The overall aim of the proposed studies is to evaluate the relationship between odor stimulation and the functioning of neuronal circuits in the olfactory bulb. The in vivo tiger salamander preparation will be studied. In previous studies, the temporal patterns of synaptic potentials and spikes elicited in single mitral/tufted cells by controlled, monitored odor stimulation were examined using intracellular recording techniques, and the latencies of the initial components of the patterns were found to decrease with increases in odor concentrations. Hyperpolarization preceded evoked spikes except in one pattern evoked by high odor concentration, unlike in responses to electrical stimulation of the olfactory nerve or tract. In the responses to electrical stimulation, excitation always preceded inhibition. In the proposed studies, sources of the odor response components of mitral/tufted cells, particularly the early hyperpolarization, will be investigated using electrophysiological, pharmacological and anatomical techniques. In the electrophysiological studies, the olfactory nerve or tract will be stimulated at various times following odor stimulation, in order to observe how odor response components are affected by the synchronous orthodromic or antidromic activation of bulbar circuits. In the pharmacological studies, after documenting the responses of a cell to electrical and odor stimulation, an injection of a GABA agonist or antagonist will be made in the glomerular, external plexiform, or mitral body layer, and responses subsequent to the injection will be examined. In the anatomical studies, recorded cells will be labelled for morphological study by intracellular injection of horseradish peroxidase, or immunocytochemical techniques will be used to examine the distribution of GABA and potential modulators of GABA-mediated inhibition in the different bulbar layers. The correlated analysis of odor responses is anticipated to provide new and significant information about the processing of odor information in olfactory bulb circuits.